For low voltage cables, stress control cones can be pre-molded in a factory and slipped over the cable in the field. Such pre-molded stress control cones are acceptable also for medium voltage cables having relatively large insulation walls with corresponding low-voltage stresses at their outside. However, they cannot be used with extruded cables of new design, which for the same voltage rating have their wall thicknesses reduced, and therefore, high stresses at their outer surfaces or for very high voltage cables. In these cases, to assure a reliable dielectric system in the termination area, it becomes necessary to assure an intimate contact between the cable insulation and the stress control cone; that is, it becomes necessary to mold the stress cone directly over the insulation without leaving any cavity or harmful contaminant between the original and newly applied insulation. The present disclosure describes a novel apparatus and method for assuring good bond between the components and good performance of the molded stress control cone.
With the utilization of the novel apparatus of the invention, the materials used in the stress cones are preferably the same as used for the extruded cable insulation system with which the stress control cones of this invention are used are molded in the field directly over the insulation, and can also be applied in a factory, on at least the cable ends located at the outside of the shipping reels, and joined in the field with the rest of the insulated cable. Using materials having the same dielectric characteristics as those prevailing in the rest of the cable provides uniformity in stress distribution and allows reduction in size.
Molded stress cones for 138 through 345 kV high-voltage stress extruded dielectric cables are not available at present. Present terminations for extruded type cables rated 138 kV consist of prefabricated stress cones, slipped over the insulation or on hand wrapped tapes which are made to conform to specific design shapes. The prefabricated stress cone type terminations of the prior art utilizes prefabricated slip-on insulation cylinders in conjunction with mechanical loading devices to compress the prefabricated units against the cable insulation. Both terminations are encased in ceramic housings and require highly skilled personnel for proper installation. The space between the cable insulation and the inner surface of the terminal in both cases is filled with an insulating fluid (typically polybutene oil) which prevents partial discharges in that area.
This means of prevention of partial discharges may be adequate at modest voltage stresses appearing at surfaces of present cables (operating below avg. stresses 100 V/mil). However, it is marginal at high-voltage stresses which exist in the high stress cables operating at average stresses of 150-200 V/mil). Both the slip-on type and the hand-wrapped stress cones are sensitive to cable diameter, to position of the components and have a limited dielectric strength. Another deficiency of the present termination is the different thermal expansion factor of its stress control cone components as compared to the thermal expansion factor of the cable insulation. This may lead to discontinuities in the insulation-shielding interface. In this event, partial discharges will develop within the termination area and premature breakdown may occur. Furthermore, this type of termination is sensitive to cable dimensions which vary with temperature. Molded stress control cones, made as an integral part of the cable insulation in accordance with this invention, prevent these deficiencies.
Molding of the stress control cones of this invention onto the cable insulation makes them an integral part of the insulation, consequently lowering the radial voltage stresses at the interface between the cone and the rest of the termination. In addition, these molded stress cones provide the following advantages:
(a) The insulation of the new stress cones is of the same material as the insulation of the cable, therefore, having similar electrical and thermal properties. Under these conditions, the characteristics of the stress cones are similar to that of the cable. This is of special importance at load cycling up to emergency temperatures (130.degree. C.) in cross-linked polyethylene cable systems operating at very high voltage stresses.
(b) The purified insulating compound used in the stress cones contains a minimum of contaminants, thus assuring high dielectric strength of the molded insulation.
(c) Constant high pressure maintained during the heating and cooling time required for molding and curing the insulation assures a uniform, void free insulation build-up.
(d) The new stress cones are much more uniform from a mechanical point of view than the currently used stress cones.
Other objects, features and advantages of the invention will appear or be described as the specification proceeds.